NEW YORK -- A day after a bombing injured 29 people in a Manhattan neighborhood, police are scouring the area for clues about who was responsible for the explosion and why it happened.

"We know it was a very serious incident, but we have a lot more work to do to be able to say what kind of motivation was behind this," New York Mayor Bill de Blasio told reporters Sunday. "Was it a political motivation? Was it a personal motivation? We do not know that yet."

A few blocks away from the blast site, investigators found one possible clue: a pressure cooker, with dark-colored wiring sticking out, connected by silver duct tape to what appears to be a cellphone, officials said.

It's still unclear whether that suspicious device has any connection to Saturday's explosion, which shook New York City's Chelsea neighborhood and sent panicked people scrambling for cover. A police bomb squad was still investigating the device Sunday afternoon, NYPD Chief of Detectives Robert Boyce said.

"We don't have everything in. Think of a jigsaw puzzle out in the street right now," he said. "Do we think they're related? Yes, we have to move in that direction, but right now we're not ready to make those calls yet."

The blast occurred on the same day an explosion went off near a Marine Corps charity run in New Jersey and a man stabbed nine people at Minnesota mall, leaving people across the country on edge. Authorities are investigating all three incidents as possible terror acts. In New York, law enforcement officials and the mayor said that without knowing who's responsible or what the motive was, it's too soon to call the Saturday bombing a terror attack.

The Department of Homeland Security is actively monitoring and participating in the investigations in New York and New Jersey. Investigators have found some similarities between the explosives used in New York and New Jersey, according to multiple law enforcement officials, but authorities said they have not concluded the incidents are linked.

"We do not have any specific evidence of a connection, but that will continue to be considered," de Blasio said. "We're not taking any options off the table."

Authorities step up security, ask public for help

City officials called on the public to help them track down who's responsible.

Investigators are also combing through surveillance video for clues, but so far they haven't found anything pointing to a suspect, a senior law enforcement official said. Some of the video is not helpful because the cameras are either too far away or the footage is too grainy to tell what's on it, the official said. Investigators will be canvasing for additional video Sunday.

Authorities have cordoned off the street where the explosion occurred, in the Chelsea neighborhood south of Midtown in western Manhattan. A small crowd watched from behind police tape Sunday as investigators gathered at the blast site. Some onlookers said residents who live on the block still hadn't been allowed to return to their homes.

Investigators are still searching the scene for evidence, officials said.

"The evidence we've collected is being taken to our lab at Quantico for review, and we are following every available lead," FBI Assistant Director in Charge William Sweeney Jr. said Sunday.

Meanwhile, New Yorkers will see an increased police presence around the city, de Blasio said.

Stepped-up security across the city is common as world leaders arrive for the United Nations General Assembly meeting, which is underway. But now it will be even more intense, de Blasio said.

"You should know you will see a very substantial NYPD presence this week -- bigger than ever," he said.

And New York Gov. Andrew Cuomo said 1,000 additional New York State Police officers and National Guard troops will be deployed to patrol bus terminals, airports and subway stations.

The increased policing, Cuomo said, is "just to err on the side of caution."

"I want New Yorkers to be confident when they go back to work on Monday that New York is up and running and we're doing everything that we need to do," he said.

Suspicious device found nearby

Police officers and federal agents searched the streets with flashlights, robots and dogs early Sunday to ensure there were no other devices in the area.

Multiple local and federal law enforcement officials described the pressure cooker found at 27th Street. None of them would say what was inside the cooker, which has since been removed from the scene.

A piece of paper with writing on it was found nearby, the officials said.

De Blasio said investigators believe there is no connection to an earlier incident in New Jersey, where an explosion went off in a garbage can on the route of a Marine Corps charity run Saturday.

The New York explosion caused "significant property damage," Cuomo said.

"We really were lucky that there were no fatalities," he said.

By Sunday morning, all 29 injured victims had been released from hospitals, the governor said.

'I could feel it in my chest'

The explosion rocked Chelsea, a New York neighborhood packed with restaurants, subway stations, shops, businesses and art galleries.

Nearby resident Sam Smith, 50, said he was closing the shades of his apartment when the blast hit.

"All I saw was a big light, and then I heard the explosion," he told CNN as he watched investigators and waited for word about whether he could return home Sunday morning. "I could feel it in my chest. It took me an hour and a half before I could hear again."

Danilo Gabrielli, 50, was watching TV at his 23rd Street apartment about a block away from the explosion site when he heard the blast. He rushed to see what had happened and found a chaotic scene.

"We smelled something, like an intense sulfur smell, and saw smoke coming out of this building. I saw pieces of metal -- not large, but not small either. A few friends of mine saw glass there."

Gabrielli said "the entire neighborhood is real scared."

"It's a real quiet neighborhood -- not like the center of the city or the Wall Street area. It's tiny bars, where you go to grab a drink, grab a bite to eat, watch a film. We were worried."

Surveillance video shows windows shattered with terrified people running on the streets and ducking. The video, which shows angles from inside and outside Orangetheory Fitness in Chelsea, shows a screen laying outside on the street after the explosion and what appears to be papers flying in the air.

Some social media users said the sound of the explosion was heard as far away as Hoboken, New Jersey, across the Hudson River.